Publication details

Zincoberaunite from Krásno, Czech Republic


TVRDÝ Jaromír PLÁŠIL Jakub ŠKODA Radek

Year of publication 2019
Type Conference abstract
MU Faculty or unit

Faculty of Science

Description Results of the complex XDR structure and chemical study of a newly obtained specimen of zincoberaunite from Krásno shed light onto peculiar behavior of Al3+ in the beraunite structures and give a better idea on hydrogen bonding scheme in beraunite related minerals. According to single-crystal X-ray data, zincoberaunite from Krásno is monoclinic, space group C2/c, with a = 20.3440(19) A, b = 5.1507(3) A, c = 19.1361(15) A, beta = 93.568(8)°, V = 2001.3(3) A3, Z = 4. The general structural architecture given in previous studies on beraunite-related structures has been confirmed. Crystal structure of studied sample contains three distinct H2O groups playing a different role: while O11(Wa) and O12(-Wa) are transformer groups with [3]O atom, O14(Wa) localized in the channels running parallel to b, is non-transformer group with [4]O atom. Two OH groups help to propagate bond-valence within the framework. Interestingly, based on site-scattering refinement, Al3+ is distributed over the M-sites without preferential ordering. Based on refined occupancies and bond-valence considerations the structural formula of studied zincoberaunite is (Zn0.81Al0.19)(OH)2(Fe0.61Al0.39)(OH)2(H2[3]O)2(Fe1.52Al0.48)(H2[3]O)2(Fe1.72Al0.28)(OH)(PO4)4(H2[4]O)2, Z = 4. Electron microprobe analyses support the obtained results. However, keeping the same cations occupancy in the M2-M4 sites, the ratio of Al3+ to Me2+ in the M1 position requires the presence of divalent cations as follows: (Fe0.24Zn0.57Al0.19)1.00(Fe3.84Al1.16)5.00[(PO4)3.88(AsO4)0.10(SiO4)0.01]4.00[O0.16(OH)4.60F0.24]5.00·6H2O. Individual zincoberaunite crystals exhibit a zonality manifested by increasing Fe and decreasing Zn and Al contents from cores to margins. Accordingly, there are observed good correlations between Zn-Al (r = 0.7), Fe-Zn (r = -0.7) and Fe-Al (r = -0.9). In the low-frequency region of the Raman spectra, the strongest peak at 997 cm–1 is attributed to the v1(PO4)3- symmetric stretching vibrations. The other main bands observed are assigned to the v3(PO4)3-antisymmetric stretching vibrations (1038 cm-1), v4(PO4)3- antisymmetric bending modes (580, 603 and 677 cm–1), dM–OH bending vibrations (843 cm–1) and metal-oxygen stretching ones (206, 247 and 316 cm–1).
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